This invention relates to enhancement in the reliability of an automatic voltage regulator apparatus which controls the output voltage of a generator to a prescribed value.
FIG. 5 shows the arrangement of the automatic voltage regulator apparatus (hereinbelow, expressed as `AVR`) in a prior art system for controlling field excitation for a rotary electric machine such as a generator to obtain a constant output voltage. Referring to FIG. 5, numeral 1 designates an AC generator which has a field coil 2. A diode 3 rectifies the output of an AC exciter (hereinbelow, expressed as `AC-EXC`) 4 which has a field coil 5. Numerals 6 and 7 designate potential transformers (hereinbelow, expressed as `PTs`). A voltage setting unit 8 (hereinbelow, termed `unit 90R`) makes use of an inductive voltage regulator, and a regulating motor 9 (hereinbelow, termed `motor 90RM`) drives the unit 90R. Diodes 10-15 constitute a three-phase full-wave rectifier circuit, a capacitor 16, and a choke coil 17 smooths ripple. Shown at numeral 18 is a variable resistor. An operational amplifier 25 has input resistors 21-23 and a feedback resistor 24. Numeral 26 indicates a field follow-up device. A motor (hereinafter, expressed as `70EM`) 27 serves to drive a manual voltage setting unit (hereinafter, expressed as `70`) 28 which makes use of an inductive voltage regulator. Shown at numeral 29 is an AC.rarw.DC inverter circuit. A contact 30 is closed when the AVR is automatic, while a contact 31 is opened when the AVR is automatic and closed when the AVR is manual. An operational amplifier 35 has input resistors 32 and 33 and a feedback resistor 34. Numeral 36 indicates a balance meter. The AVR further comprises ignition circuits 41 and 42, thyristors 43-54, circuit breakers 55 and 56, and a magnet generator 57. It also comprises a shunt 61, an amplifier 62, a resistor 63, capacitors 64 and 65, and a variable resistor 66. Also comprised is an operational amplifier 69 which has resistors 67 and 68.
Next, the operation of the prior-art AVR will be explained.
When the AVR is to be started, the contact 30 is opened, and the unit 70E (28) is set at a no-load prescribed voltage, the signal of which is provided. Outputs proportional to the signal are delivered from the two thyristor circuits 43-48 and 49-54, and are used to excite the AC-EXC 4. Using an output from the AC-EXC 4, the generator 1 is excited.
In case of putting the AVR into the automatic mode, the unit 90R (8) is regulated to acknowledge it with the balance meter 36 that the output of the operational amplifier 25 being an AVR signal is null, and the contact 30 is thereafter closed.
The output voltage of the generator 1 is stepped down to 110 V by the PT 6 and the PT 7, and is further regulated by the unit 90R (8). The resulting voltage is rectified by the three-phase full-wave rectifier circuit 10-15, and the rectified voltage is smoothed into a plus voltage. On the other hand, a voltage -E regulated to a minus fixed value is applied as the reference voltage of the operational amplifier 25. When the output voltage of the generator 1 is a prescribed voltage, the difference of the aforementioned two input voltages of the amplifier 25 is null, and hence, the output of the amplifier 25 becomes null. The field current of the AC-EXC 4 is detected by the shunt 61, and is amplified by the amplifier 62. The amplified voltage is applied across a series circuit consisting of the resistor 63 and the capacitor 64, and is differentiated by a circuit consisting of the capacitor 65 and the variable resistor 66. Thus, the operational amplifier 25 is supplied with a signal which is negatively fed back to the generator 1 and which prevents hunting from developing in the generator 1. The operational amplifier 69 is an amplifier for polarity inversion.
The field follow-up device 26 operates as follows: The output value of the unit 70E (28) is regulated beforehand so as to ensure the magnitude of the basic part of an excitation current, whereby even when the contact 30 is opened on account of the fault of the AVR attributed to any abnormality, the output voltage of the generator 1 is prevented from fluctuating greatly. To this end, the output of the operational amplifier 25 whose output corresponds to the deviation of the plurality of inputs is normally monitored with the balance meter 36, and the field follow-up device 26 causes the unit 70E (28) to follow up the output voltage of the generator 1 so that the output of the amplifier 25 may become null. Of course, the device 26 has some operational dead zone and does not effect the follow-up within the dead zone. Moreover, even when the dead zone has been exceeded, the motor 70EM (27) does not drive the unit 70E (28) without the lapse of a certain fixed time interval since then. In this manner, the unit 70E (28) is endowed with the characteristic according to which it follows up the output voltage of the generator 1 with the delay of the fixed time interval.
When the output voltage of the generator 1 changes fast, the operational amplifier 25 produces an output transiently, and the sum of this output with the output of the unit 70E (28) is used to control the field current. Thus, the generator voltage is controlled so as to hold a fixed value.
Each of the two thyristor circuits 43-48 and 49-54 has a capacity enough to excite the field coil 5 of the AC-EXC 4. The dualized arrangement of the two thyristor circuits ensures the operation of the system even when either circuit has failed to ignite, and the outputs of these thyristor circuits are connected by the circuit breakers 55 and 56. In the case where either thyristor circuit has gone wrong, it can be repaired by opening the breaker on its side.
Now, when the generator voltage has risen, the plus voltage obtained by the three-phase full-wave rectification becomes greater in the absolute value than the minus reference voltage, and a lowering signal is provided from the amplifier 25. This signal retards the ignition phases of the ignition devices 41 and 42, to decrease the outputs of the thyristor circuits 43-48 and 49-54 and to decrease the field current of the coil 5. Thus, the output voltage of the generator 1 is controlled so as to become the prescribed value.
Conversely, when the generator voltage has fallen, the minus input of the amplifier 25 becomes greater to generate a raising signal therefrom, and the output of the amplifier 35 increases. Then, the ignition phases of the ignition devices 41 and 42 advances to increase currents from the thyristor circuits and to increase the excitation current. Thus, the output voltage of the generator 1 is corrected to the prescribed value.
In addition, the voltage setting of the unit 90R (8) is performed by changing the proportion between the input voltage and output voltage thereof. More specifically, when the proportion of input voltage to output voltage is increased, the generator voltage must be increased in order to keep constant the plus DC voltage produced by the threephase full-wave rectification. This is equivalent to having set the generator voltage to be greater. On the other hand, the decrease of the proportion is equivalent to setting a smaller generator voltage.
The prior-art AVR described above has the disadvantage that, when either of the two thyristor circuits has broken down in the direction of advancing the ignition phase, the output voltage of this thyristor circuit increases to abruptly raise the output voltage of the generator, so the generator voltage might become an overvoltage. Another disadvantage is that, in such a case, the generator must be shut down during the period that the broken down thyristors are being repaired.